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El Niño may skew the balance of marine ecosystem
Like grass in a terrestrial ecosystem, microscopic phytoplankton are the primary energy producers for life in the ocean.
Besides providing sustenance for diverse organisms, phytoplankton also absorb large amounts of the carbon dioxide believed to contribute to global warming, scientists say.
As with most members of the food chain, a fine balance of phytoplankton is necessary for a healthy marine environment. Denied the proper amount of the energy source, organisms starve and their reproduction rates plummet. With too much phytoplankton, harmful algal blooms form, depleting oxygen and killing marine life.
The El Niño weather phenomenon may skew the equilibrium of phytoplankton, scientists from the Scripps Institution of Oceanography report in the Sept. 15 issue of Geophysical Research Letters.
Using high-resolution, color-sensitive satellite images, the researchers examined the impact of the 1997/1998 El Niño event on the abundance of phytoplankton in the California ocean current system.
Launched in 1996 and 1997, the satellites enabled the researchers to monitor the amount of phytoplankton in the top 10 meters of the ocean.
"Our work adds to a growing body of evidence about the impact of El Niño on the ocean ecosystem," said Greg Mitchell, a research biologist in the marine research division at Scripps.
In southern and central California, ocean temperatures rose as much as four degrees higher than normal during the height of El Niño, Mitchell explained. This resulted in a critical reduction of surface nutrients and consequently a reduction in the high-concentration patches of phytoplankton that may be necessary for the reproductive success of many fish populations.
"Success of fish population recruitment, and therefore commercial fisheries, may in part depend on inter-annual cycles of nutrient and phytoplankton distributions associated with El Niño and La Niña," he said.
"(The 1997/1998 El Niño year) was not a good year for fisheries," said John Hunter, director of the fisheries division at NOAA's Southwest Fisheries Science Center.
Most affected were organisms with shorter life spans that depend on cold water ecosystems, he explained. "Squid, which have a life span of about six months virtually disappeared during El Niño."
For other species such as rockfish, which can live up to 10 years, the effects of El Niño may be more subtle. "Most animals are built to withstand one bad year," Hunter said.
The researchers also found a significant increase in phytoplankton off Baja California.
While Mitchell said the increased levels of phytoplankton may have been helpful rather than harmful to organisms in the region, excessive reproduction of phytoplankton results in algal blooms that can cause oxygen depletion in the ocean. The depletion can suffocate other aquatic life and generally degrade the environment.
"If we can learn more about how the physical force of the ocean influences fish yields, perhaps we can better manage our fisheries," he said.
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